1. Field of the Invention
This invention relates to an acryl-based pressure-sensitive adhesive composition.
2. Description of the Prior Art
Recently an acryl-based pressure-sensitive adhesive has widely been used in place of conventional natural rubber based and synthetic rubber based pressure-sensitive adhesives, because of its excellent adhesion characteristics and durability. See, for example, U.S. Pat. No. 2,973,286. This acryl-based pressure-sensitive adhesive is composed mainly of an acryl-based polymer which has sticking properties at room temperature and which is generally produced by the solution polymerization of an acryl monomer, such as acrylate, methacrylate, etc., if necessary, in combination with a modifying monomer, such as acrylic acid, maleic acid, acrylamide, styrene, vinyl acetate, etc.
Such an acryl-based polymer, however, has generally poor cohesive strength and, therefore, it has usually been required to be crosslinked by use of a crosslinking agent, e.g., a phenol resin, a melamine compound, an isocyanate compound, an epoxy compound, metallic ions, etc. Such crosslinking or modification give rise to problems in respect of quality and productivity, for example, some of such crosslinking agents reduce the moisture resistance and heat resistance of the adhesive. Also, the drop in adhesion strength owing to the improvement in cohesive strength owing to the improvement in cohesive strength and the heat energy required for the crosslinking are sometimes significant; and the selection of a suitable crosslinking agent and the determination of the appropriate amount thereof to be added require much time and labor.
It has, therefore, been the subject of long and continuing research to develop novel acryl-based pressure-sensitive adhesives which are able to provide both high cohesive strength and high adhesive strength without using crosslinking agents.